Reactive deposition of metal thin films within porous supports from supercritical fluids

Continuous palladium films were synthesized at controlled depths within porous alumina disks by H₂ reduction of organopalladium compounds dissolved in supercritical CO₂ at 60 °C using an opposing reactants deposition geometry. Film position was controlled by adjusting the relative concentrations of H₂ and the palladium precursor (π-2-methylallyl(cyclopentadienyl) palladium (II) or palladium(II) hexafluoroacetylacetonate) on opposite sides of the alumina substrate. Because of a disparity in the diffusivity of the metal precursor and H₂ in the support, a temporary barrier of poly-4-methyl-1-pentene on the H₂ side of the alumina substrate was used to reduce H₂ flux in a controlled manner. Guided by a simple mass transport model, Pd films between 2 and 80 μm thick were deposited at prescribed depths between 80 and 600 μm as measured from the precursor side. Electron probe microanalysis indicated complete pore filling of the porous alumina at the reaction zone and X-ray diffraction revealed that the structure of the deposit is nanocrystalline. The flux of N₂ through the alumina disk was reduced by over 4 orders of magnitude after deposition and annealing at 500 °C.